1. Field of the Invention
The present invention is related to an active harmonic suppression equipment and control method therefor, and it is particularly applied in parallel connection with non-linear loads, so as to suppress harmonic current generated by nonlinear loads.
2. Description of the Related Art
Recently, the characteristics of power electronic devices have been improved significantly. The power electronic devices with high-voltage rating, high-current rating, and high switching speed have been developed due to the improvement of semiconductor manufacturing technique. Power electronic devices are applied in electric power equipment, such as switching power supply, uninterruptible power supply, motor driver, arc furnace, trolley car, battery charger, and lighting appliance etc. The electric power equipment may generate a large amount of harmonic current due to the nonlinear input characteristic of such loads. The harmonic current will pollute the power system, and it results in the problems such as transformer overheat, rotary machine vibration, degrading voltage quality, electric power components destruction, medical facilities malfunction etc. In order to improve the problem of harmonic pollution effectively, many harmonic limitation standards, such as IEEE519-1992, IEC1000-3-2, and IEC1000-3-4 etc., have been established. Therefore, how to solve the harmonic problem is an important topic in the power system today.
Conventionally, the passive power filter is used to solve the problems of harmonic. The passive power filter is consisted of some passive elements, such as inductor, capacitor and resistor. The tuner filter is the widely used passive power filter, and it can attenuate the harmonic current near the tuning frequency. The passive power filter has the disadvantages that
(1) sensitive to the variation of utility impedance,
(2) sensitive to frequency variation,
(3) having the risk of series/parallel resonance,
(4) fixed filtering frequency.
The most serious of the above disadvantages is the series/parallel resonance. The series/parallel resonance may result in over-current/over-voltage of inductor and capacitor, and it will damage the passive power filter. Since the utility impedance has a significant effect on the performance of passive power filter, it is very hard to obtain an excellent filter performance in practical application. Moreover, the harmonic currents produced by neighboring nonlinear may flow into the passive power filter and result in the overload of the passive power filter.
Recently, the harmonic suppression equipment based on power electronic technique has been developed. This harmonic suppression equipment can suppress the different order harmonic components of the nonlinear loads simultaneously using only the same harmonic suppression equipment. It is named as an active power filter. FIG. 1 illustrates a system single-line diagram of a traditional active power filter. Referring to FIG. 1, this diagram includes a filter inductor, a power converter, and a dc capacitor. The power converter is used to generate a compensating current via the filter inductor to inject into a power feeder. The role of the filter inductor is used for suppressing high frequency ripple current due to the switching behavior of the power converter. The inductance of filter inductor depends on the factors of switching frequency, dc voltage and ripple current. The dc capacitor located in the dc bus of power converter is operated as an energy buffer. Although, the traditional active power filter is capable of suppressing harmonic, it has the following disadvantages:
(1). In order to suppress the ripple current from the power converter, the filter inductor with a large inductance is used.
(2). A higher dc bus voltage of power converter is required, and it results in the high voltage rating of dc capacitor and power electronic devices.
(3). Using a larger filter inductor will result in the larger power loss, poor energy efficiency, more heat dissipation, bulk dimension and weight.
(4). Using a larger filter inductor also results in the degrading of the high frequency response.
The present invention intends to provide a new active harmonic suppression equipment which improves the above-mentioned problems of the traditional passive power filter and the traditional active power filter. The inventive active harmonic suppression equipment comprises a power converter, a capacitor/inductor set in which the capacitor is connected in serial with the inductor, a C-R-C filter set, a dc capacitor bank, and a control circuit. The inventive active power filter can reduce the dc bus voltage of power converter significantly. Hence, it can reduce the voltage rating of the dc capacitor and power electronic devices as well as the inductance of the inductor. The inventive active power filter generates harmonic current which is opposite to that of nonlinear load, and it also supply a constant reactive power according to the capacitance of the capacitor in the capacitor/inductor set. The fundamental component of feeder voltage drops mainly in the capacitor of the capacitor/inductor set, and the power converter only generates harmonic voltage. Then, the dc bus voltage of power converter can be reduced and the ripple amplitude of power converter output current is also reduced. Hence, the inductance of the capacitor/inductor set will be smaller due to the smaller ripple current. It means that the high frequency response of the inventive active power filter is better than that of the traditional active power filter. The salient benefits of the inventive active power filter are the low voltage rating of dc capacitor, low voltage rating of power electronic devices, smaller inductor, and the better filtering performance.
The primary objective of this invention is to provide a new active harmonic suppression equipment and control method therefor which comprises a power converter, a capacitor/inductor set in which the capacitor and inductor are serial link, a C-R-C filter set, a de capacitor bank and a control circuit. The power converter converts a dc voltage to a compensating voltage which is further converted into a compensating current by the capacitor/inductor set and a C-R-C filter set, and then is injected into a power feeder to cancel harmonic currents generated by nonlinear loads so that the utility current waveform is approached to non-distorted sinusoid.
The present invention is an active harmonic suppression equipment and control method therefor. The active harmonic suppression equipment comprises, a power converter, a serially connected capacitor/inductor set, a C-R-C filter set, a dc capacitor bank and a control circuit. The dc capacitor bank is used as an energy storage device, and the power converter electrically connected thereto is used to convert the dc voltage of dc capacitor bank into a compensating voltage. The inductor in a serially connected capacitor/inductor set is used to smooth the waveform of power converter output voltage, and the capacitor in a serially connected capacitor/inductor set is used to supply the reactive power. The C-R-C filter set is used to filter out the high frequency ripple due to the switching operation of power converter. The serially connected capacitor/inductor set and the C-R-C filter set are converting the compensating voltage into a compensating current, and it is injecting into power feeder in order to filter harmonic currents generated by nonlinear loads. Then, the utility current is approximated to be sinusoidal.
The control method for the inventive active harmonic suppression equipment detects the load current, the utility current, the output current of power converter and the voltage of the dc capacitor bank to calculate an expected compensating voltage of the power converter. Then, the expected compensating voltage of the power converter is sent to the pulse width modulator, and the output signals of pulse width modulator are used to drive the power switching devices of the power converter to generate the compensating voltage. Then the compensating voltage is converted into a compensating current via a serially connected capacitor/inductor set and a C-R-C filter set, and the compensating current is injected into the power feeder in order to filter the harmonic currents generated by nonlinear loads. Then, the utility current is approximated to be sinusoidal.
Other objectives, advantages and novel features of this invention will become more apparent from the following detailed description and the accompanying drawings.